The present invention relates to the general subject of electronic ballasts for fluorescent lamps and more particularly to a single switch inverter based electronic ballast.
Electronic ballasts for fluorescent and other gas discharge lamps are well known. Electronic ballasts operate at much higher frequencies and are more energy efficient than conventional line frequency ballasts. Electronic ballasts can reduce the energy consumption of a lighting system by more than 20%. Higher frequency operation provides for the same amount of light at a lower input power.
Electronic ballasts having a dimming function are also well known. Dimming, in combination with the energy efficient characteristics of high frequency operation of the lamp, can result in further energy savings.
Although the energy efficient characteristics of electronic ballasts are attractive, their production cost affects the commercialization of electronic dimming ballasts. A major factor contributing to the cost of producing electronic ballasts is the number of parts required for the ballast. Line frequency ballasts require fewer parts and, therefore, are less costly to produce.
In addition, since line frequency ballasts have been known for over fifty years, they are highly optimized and exhibit fewer problems affecting their performance and reliability. Electronic ballasts on the other hand, with their greater number of parts, exhibit more performance problems. Further, having a greater number of parts means that the electronic ballast is more susceptible to failure.
Many known electronic ballasts use two or more power semiconductor switching devices in their inverter circuits. These switching devices dissipate a significant amount of heat in operation, which may adversely affect the reliability of the ballast and generally require heat sinking to the ballast enclosure. In addition, power semiconductor switching devices are expensive, and thus significantly add to the total cost of the ballast.
A typical topology for a conventional electronic ballast uses a half bridge inverter circuit containing two semiconductor switching devices such is two metal oxide semiconductor field effect transistors (MOSFET). Such a circuit is described in above noted co-pending application Ser. No. 10/006,021. The top switch in this conventional configuration requires a high-side driver circuit because it""s control terminal is not referenced to the circuit common. The high side driver may be a transformer or an integrated circuit such as IR2111 chip driver sold by the International Rectifier Corporation of El Segundo, Calif. In addition to the high side driver, the half bridge circuits in conventional pulse width modulated (PWM) electronic ballasts also require blocking diodes and fast recovery free wheeling diodes to prevent the conduction of the intrinsic body in the switches.
Other prior art electronic ballasts can additionally include active power factor correction circuits to improve ballast input current total harmonic distortion. Active power factor correction circuits are often implemented with a boost converter type circuit. An example of a ballast employing a boost converter is described in xe2x80x9cSingle-Switch Frequency-Controlled Electronic Dimming Ballast With Unity Power Factor,xe2x80x9d Chang-Shiarn Lin et al., IEEE Transactions on Aerospace and Electronic Systems, pages 1001-1006, July 2000.
An additional disadvantage of prior art ballasts is a characteristic in-rush of current into the ballast when AC power is applied to the ballast. Typical ballasts include a large storage capacitor which is charged when AC power is applied to the ballast. The current to charge this storage capacitor can be many times larger than the typical nominal input current of the electronic ballast. This large in-rush of current can cause damage to the equipment energizing the electronic ballast. In order to avoid this large in-rush of current, many ballasts include additional circuitry to limit this current. This additional circuit increases the cost and complexity of the ballast. It would be advantageous to have a ballast that inherently limits the in-rush current without additional circuitry whose sole function is to limit in-rush current.
It would be desirable to have an electronic ballast circuit that contains fewer parts to reduce cost and increase reliability.
An important indicator of lamp current quality for a gas discharge lamp such as a fluorescent lamp is the current crest factor (CCF) of the lamp current, which is defined as the peak to RMS (root mean square) ratio of the lamp current.                     CCF        =                              I            PK                                I            RMS                                              (                  Equation          ⁢                      xe2x80x83                    ⁢          1                )            
A low CCF is preferred because a high CCF can cause the deterioration of the lamp filaments which would subsequently reduce the life of the lamp. A CCF of 2.1 or less is recommended by Japanese Industrial Standard (JIS) JIS C 8117-1992, and a CCF of 1.7 or less is recommended by the International Electrotechnical Commission (IEC) Standard 921-1988-07.
In an AC power system, the voltage or current wave shapes may be expressed as a fundamental and a series of harmonics. These harmonics have some multiple frequency of the fundamental frequency of the line voltage or current. Specifically, the distortion in the AC wave shape has components which are integer multiples of the fundamental frequency. Of particular concern are the harmonics that are multiples of the 3rd harmonic. These harmonics add numerically in the neutral conductor of a three phase power system. Total harmonic distortion (THD) of the ballast input current is preferred to be below 33.3% to prevent overheating of the neutral wire in a three phase power system. Further, many users of lighting systems require ballasts to have a ballast input current total harmonic distortion of less than 20%.
It is also desirable to reduce or eliminate the very high frequency harmonics of the output waveform of the electronic ballast in order to reduce the electromagnetic interference (EMI) emissions of the ballast.
In accordance with a first feature of the invention, an electronic ballast for driving a gas discharge lamp includes a rectifier to convert an AC line input voltage to a rectified voltage, a valley fill circuit including an energy storage device such as a capacitor, the energy in this device being used to fill the valleys between successive rectified voltage peaks to produce a valley filled voltage, and an inverter circuit having a single controllably conductive device to convert the valley filled voltage to a high-frequency AC voltage. The energy storage device can be a capacitor or an inductor or any other energy storage component or combination of components. Charging the energy storage device refers to increasing the energy stored in the energy storage device. A controllably conductive device is a device whose conduction can be controlled by an external signal. These include devices such as metal oxide semi-conductor field effect transistors (MOSFETs), insulated gate bi-polar transistors (IGBTs), bi-polar junction transistors (BJTs), triacs, SCRs, relays, switches, vacuum tubes and other switching devices. The high frequency AC voltage is used for driving a current through a gas discharge lamp. A control circuit controls the conduction of the controllably conductive device in a novel way to deliver a desired lamp current to the gas discharge lamp and draw an input ballast current with a reduced total harmonic distortion. The electronic ballast of the invention described can drive more than one gas discharge lamp.
According to an additional aspect of the ballast of the present invention, the inverter circuit includes a single controllably conductive device such as a power MOSFET. The power MOSFET may be connected to the second winding of a transformer. The conduction of the MOSFET alternately connects and disconnects the second winding of the transformer to the output of the valley fill circuit. A suitable control circuit is used to control the controllably conductive device.
Still another aspect of the invention involves the coupling of a first winding with the second winding of the transformer. When the second winding is connected to the valley fill circuit via the single controllably conductive device, the first winding is disconnected from the valley fill circuit by a reverse biased diode. When the single controllably conductive device is in the non-conducting state, some of the energy stored in the magnetizing inductance of the transformer is transferred to the load via the first winding or a third winding, and some of the energy is transferred to a capacitor of the valley fill circuit so as to recharge this valley fill capacitor. This transfer of energy to the valley fill capacitor has two purposes. First, the capacitor is recharged for use during the valley of the rectified line voltage. Second, the capacitor establishes a fixed voltage across the first winding. The capacitor is adequately large with respect to the high frequency operation of the inverter such that its average voltage does not change significantly during a single high frequency cycle. This, in a high frequency sense, makes the capacitor look like a voltage source to the first winding. This in turn establishes a fixed voltage on the second winding via the turns ratio between the first winding and the second winding. Setting this predetermined voltage on the second winding of the transformer establishes the off-state voltage stress applied to the single controllably conductive device.
A yet further aspect of the invention involves using a valley fill circuit to prevent the voltage supplied to the inverter circuit from dropping to zero when the rectified input line voltage reaches a minimum value. The valley fill circuit comprises an energy storage device such as a capacitor. The valley fill circuit capacitor does not charge from the rectified line directly; rather, it charges indirectly via a tap on the first winding of the transformer. The capacitor is prevented from discharging into the first winding by a diode. A current limiting resistor may be employed to limit the amount of current that flows from the first winding into the valley fill capacitor.
Another aspect of the ballast is the operation of the control circuit used to control the controllably conductive device. The control circuit reduces the conduction time of the controllably conductive device at the time near the peak of the AC line voltage, and thereby reducing the current crest factor of the lamp current from that which would normally have occurred.
Still another aspect of the invention involves a current drawing circuit to supplement the ballast input current in order to increase the length of time during which current may be drawn from the AC line to improve ballast input current total harmonic distortion. The current drawing circuit may be a cat ear circuit which draws current during a predetermined period, for example, at the beginning and end (or one of them) of an AC line voltage half cycle. The cat ear circuit may also be used to provide power for the control circuit of the inverter circuit.
Still another aspect of the ballast of the invention includes a coupling impedance that connects the inverter circuit to a gas discharge lamp. Typically this impedance is an inductor or a tank circuit. The operation of the controllably conductive device causes the inverter transformer to supply a high frequency AC voltage which is applied to the connected lamp through the coupling impedance. The impedance reduces the harmonic content of the output current thereby reducing the EMI emissions of the ballast.
An electronic ballast according to the present invention includes fewer parts and is, thus, more reliable and less costly, has a low CCF of 2.1 or lower, preferably 1.7 or lower; has a low THD of 33.3% or lower, preferably 20% or lower; and has reduced EMI emissions. These and other advantageous aspects of the present invention will be explained in detail below with reference to the drawings.